1. Field of the Invention
The present application relates to a differential circuit, in particular, a differential amplifier compensating undershoots appearing in outputs thereof.
2. Related Background Art
It has been known in the field that a differential circuit comprised of bipolar junction transistors (hereafter denoted as BJT) usually accompanies with undershoot in falling edges due to stray capacitors of the BJT. When such a differential circuit drives an optical modulator, the undershoot brings degraded optical output.
A Japanese Patent application published as JP-H08-102622A has disclosed a differential amplifier that suppresses undershoots. FIG. 9 shows a circuit diagram of a differential amplifier disclosed therein where a discharged current of the junction capacitor Cbe between the base and emitter of transistors Q1 (Q2) is fed back in collector current of the other transistor Q2 (Q1) through a driving transistor Q5 (Q6). Thus, the discharge current is compensated from flowing in load resistors, and undershoots appearing in the output due to the current flowing in the load resistor are suppressed.
However, an additional transistor Q5(Q6) is necessary in the conventional circuit shown in FIG. 9, which needs to raise the power supply voltage by at least a base-emitter voltage Vbe, which is typically about 0.8 V. Thus, the power consumption of the amplifier increases.